Catalyst for decomposition of sulfur trioxide and hydrogen production process

ABSTRACT

To provide a sulfur trioxide decomposition catalyst, particularly, a sulfur trioxide decomposition catalyst capable of lowering the temperature required when producing hydrogen by an S—I cycle process. 
     A sulfur trioxide decomposition catalyst comprising a composite oxide of vanadium and at least one metal selected from the group consisting of transition metal and rare earth elements is provided. Also, a sulfur dioxide production process comprising decomposing sulfur trioxide into sulfur dioxide and oxygen by using the sulfur trioxide decomposition catalyst above, is provided. Furthermore, a hydrogen production process, wherein the reaction of decomposing sulfur trioxide into sulfur dioxide and oxygen by an S—I cycle process is performed by the above-described sulfur dioxide production process, is provided.

TECHNICAL FIELD

The present invention relates to a catalyst for decomposition of sulfur trioxide (SO₃). The present invention also relates to a hydrogen production process including a step of decomposing sulfur trioxide by using the catalyst for decomposition of sulfur trioxide.

BACKGROUND ART

In recent years, because of problems such as global warming, hydrogen is attracting attention as a clean energy involving no formation of carbon dioxide during combustion.

In general, for the production of hydrogen, steam reforming of a hydrocarbon fuel, represented by the following formulae (A1) and (A2), is employed: C_(n)H_(m) +nH₂O→nCO+(n+m/2)H₂  (A1) CO+H₂O→CO₂+H₂  (A2) C_(n)H_(m)+2nH₂O→nCO₂+(2n+m/2)H₂  Total reaction:

Accordingly, although carbon dioxide is not formed by the combustion itself of hydrogen, carbon dioxide is generally formed in the production of hydrogen.

In this connection, use of solar thermal energy or nuclear thermal energy has been proposed as the method for producing hydrogen without using a hydrocarbon fuel (see, Patent Document 1 and Non-Patent Document 1).

As the method for producing hydrogen from water by utilizing thermal energy, there has been proposed a method called an S—I (sulfur-iodine) cycle process represented by the following formulae (B1) to (B3): H₂SO₄(liquid)→H₂O(gas)+SO₂(gas)+½O₂(gas)  (B1) (reaction temperature=about 950° C.,ΔH=188.8 kJ/mol-H₂) I₂(liquid)+SO₂(gas)+2H₂O(liquid)→2HI(liquid)+H₂SO₄(liquid)  (B2) (reaction temperature=about 130° C.,ΔH=−31.8 kJ/mol-H₂) 2HI(liquid)→H₂(gas)+I₂(gas)  (B3) (reaction temperature=about 400° C.,ΔH=146.3 kJ/mol-H₂)

The total reaction of the S—I (sulfur-iodine) cycle process represented by formulae (B1) to (B3) is as follows: H₂O→H₂+½O₂ (ΔH=286.5 kJ/mol-H₂(on higher heating value basis)) (ΔH=241.5 kJ/mol-H₂(on lower heating value basis))

Here, the reaction of formula (B1) can be divided into two elementary reactions of the following formulae (B1-1) and (B1-2): H₂SO₄(liquid)→+H₂O(gas)+SO₃(gas)  (B1-1) (reaction temperature=about 300° C.,ΔH=90.9 kJ/mol-H₂) SO₃(gas)→SO₂(gas)+½O₂(gas)  (B1-2) (reaction temperature=about 950° C.,ΔH=97.9 kJ/mol-H₂)

That is, in the case of producing hydrogen by an S—I cycle process, the sulfur trioxide (SO₃) decomposition reaction of formula (B1-2) requires a highest temperature, and the high temperature required in this reaction cannot be easily obtained.

With respect to such a problem, in Non-Patent Document 1, a natural gas is burned, if desired, while using solar thermal energy as the heat source, whereby additional thermal energy is obtained.

Also, in order to lower the temperature required in the sulfur trioxide decomposition reaction of formula (B1-2), use of a platinum catalyst has been proposed. However, it is known that when a platinum catalyst is used in this reaction, the catalyst may have high performance at the start of use but the platinum is oxidized with oxygen produced in the reaction and due to coarsening of the platinum particle, the catalytic activity is reduced. Furthermore, the platinum catalyst is expensive and its use on an industrial scale is difficult.

In this connection, Non-Patent Document 2 has proposed a technique where in order to lower the temperature required in the sulfur trioxide decomposition reaction, a catalyst selected from the group consisting of platinum (Pt), chromium (Cr), iron (Fe) and oxides thereof is used by depositing it on an alumina support.

In addition, with respect to the S—I cycle process, Patent Document 2 has proposed a technique where in the reaction represented by formula (B2), that is, in the reaction of obtaining hydrogen iodide and sulfuric acid from iodine, sulfur dioxide and water, the reaction of sulfur dioxide with water is performed on the cathode side of a cation-exchange membrane, and the reaction of iodine is performed on the anode side of the cation-exchange membrane, whereby the subsequent separation operation is omitted.

Incidentally, other than the S—I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, a Los Alamos science laboratory cycle process and the like are known as the method for producing hydrogen by utilizing thermal energy, however, also in these processes, sulfur trioxide must be decomposed into sulfur dioxide and hydrogen as in formula (B1-2).

RELATED ART Patent Document

-   [Patent Document 1] Kokai (Japanese Unexamined Patent Publication)     No. 2007-218604 -   [Patent Document 2] Kokai No. 2005-041764

Non-Patent Document

-   [Non-Patent Document 1] A. Giaconia, et al., International Journal     of Hydrogen Energy, 32, 469-481 (2007) -   [Non-Patent Document 2] H. Tagawa, et al., International Journal of     Hydrogen Energy, 14, 11-17 (1989)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention provides a sulfur trioxide decomposition catalyst, particularly, a sulfur trioxide decomposition catalyst capable of lowering the temperature required when producing hydrogen from water.

Means to Solve the Problems

As a result of intensive studies, the present inventors have accomplished the present invention described below.

<1> A sulfur trioxide decomposition catalyst comprising a composite oxide of vanadium and at least one metal selected from the group consisting of transition metal and rare earth elements.

<2> The catalyst as described in <1> above, wherein the at least one metal is selected from the group consisting of copper (Cu), chromium (Cr), titanium (Ti), zirconium (Zr), lanthanum (La), cerium (Ce), neodymium (Nd) and a combination thereof.

<3> The catalyst as described in <1> or <2> above, wherein in the composite oxide, the atom ratio between the at least one metal and vanadium is from 1:9 to 9:1.

<4> The catalyst as described in <1> above, wherein the at least one metal is cerium (Ce), and

in the composite oxide, the atom ratio (cerium:vanadium) between cerium and vanadium is from 0.4:1 to less than 1:1.

<5> The catalyst as described in <4> above, wherein in the composite oxide, the atom ratio (cerium:vanadium) between cerium and vanadium is from 0.8:1 to 0.95:1.

<6> The catalyst as described in any one of <1> to <5> above, wherein the composite oxide is supported on a support.

<7> The catalyst as described in <6> above, wherein the support is selected from the group consisting of silica, alumina, zirconia, titania and a combination thereof.

<8> The catalyst as described in <7> above, wherein the support is a porous silica support having a pore structure.

<9> The catalyst as described in <8> above, wherein the composite oxide is supported inside the pore structure of the porous silica support, and

in the pore distribution of the porous silica support, the peak attributable to a gap between primary particles of silica is present in the range of a pore size of 5 to 50 nm and the peak attributable to the pore structure inside the silica particle is present in the range of a pore size of 1 to 5 nm.

<10> A process for producing the catalyst described in any one of <6> to <9> above, comprising:

causing the support to absorb one aqueous solution out of an aqueous solution containing a salt of a transition metal element or rare earth element and an aqueous solution of a vanadium salt, then drying and calcining the support, causing the support to absorb another aqueous solution out of an aqueous solution of a salt of a transition metal element or rare earth element and an aqueous solution of a vanadium salt, then drying and calcining the support, and firing the obtained support.

<11> A sulfur dioxide production process comprising decomposing sulfur trioxide into sulfur dioxide and oxygen by using the sulfur trioxide decomposition catalyst described in any one of <1> to <9> above.

<12> The process as described in <11> above, wherein the decomposition is performed at a temperature of 700° C. or less.

<13> A hydrogen production process comprising splitting water into hydrogen and oxygen, wherein the process comprises decomposing sulfuric acid into water, sulfur dioxide and oxygen through a reaction represented by the following formula (X1) and out of the elementary reactions represented by the following formulae (X1-1) and (X1-2) which are an elementary reaction of the reaction represented by the following formula (X1), the elementary reaction of formula (X1-2) is performed by the process described in <11> or <12> above: H₂SO₄→H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂  (X1-2)

<14> The hydrogen production process as described in <13> above, which is an S—I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, or a Los Alamos science laboratory cycle process.

Effects of the Invention

According to the sulfur trioxide decomposition catalyst of the present invention, the temperature required for the sulfur trioxide decomposition reaction can be lowered. Also, according to the sulfur dioxide production process of the present invention, sulfur trioxide can be decomposed at a relatively low temperature to obtain sulfur dioxide. Furthermore, according to the hydrogen production process of the present invention, water can be splitted at a relatively low temperature to obtain hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the results of X-ray diffraction analysis on the composite metal oxide used as an unsupported catalyst in (a) Example A1 and (b) Example A2.

FIG. 2 is a view showing the results of X-ray diffraction analysis on the composite metal oxide used as an unsupported catalyst in (a) Example A3, (b) Example A4 and (c) Comparative Example A1.

FIG. 3 is a view showing the apparatus used far evaluation of the sulfur trioxide decomposition catalyst.

FIG. 4 is a schematic view showing the catalyst surface atmosphere when (a) a cerium-vanadium oxide or (b) a lanthanum-vanadium oxide is used as the sulfur trioxide (SO₃) decomposition catalyst.

FIG. 5 is a view showing the achievement ratio based on the equilibrium conversion when a cerium-vanadium oxide is used as the sulfur trioxide (SO₃) decomposition catalyst.

FIG. 6 is a view showing the results of X-ray diffraction (XRD) analysis on the cerium-vanadium oxide (Ce/V ratio: 85 mol %).

FIG. 7 is a view showing the achievement ratio based on the equilibrium conversion when a cerium-vanadium oxide, a lanthanum-vanadium oxide, or a neodymium-vanadium oxide is used as the sulfur trioxide (SO₃) decomposition catalyst.

MODE FOR CARRYING OUT THE INVENTION

(Sulfur Trioxide Decomposition Catalyst)

The sulfur trioxide decomposition catalyst of the present invention contains a composite oxide of vanadium and at least one metal selected from the group consisting of transition metal and rare earth elements.

According to the sulfur trioxide decomposition catalyst of the present invention, the temperature required for the sulfur trioxide decomposition reaction of formula (B1-2) can be lowered and, for example, at a temperature of about 700° C. or less, the sulfur trioxide decomposition reaction can be made to proceed at a substantive rate.

As described above, in the conventional process of decomposing sulfur trioxide, a temperature near 1,000° C. is generally employed. However, the material capable of withstanding such a high temperature is very limited and moreover, is fairly expensive

Also, the high temperature close to 1,000° C. can be hardly obtained at a low cost from the solar energy. More specifically, for example, as the collector for obtaining solar thermal energy, a parabolic dish-type collector, a solar tower-type collector and a parabolic trough-type collector are known. Of these, in the parabolic trough-type collector enjoying a simple structure and a low cost and being suitable for a large-scale plant, collection of solar energy at a high temperature close to 1,000° C. is unrealistic in view of balance between collection of solar energy and dissipation of energy due to radiation.

Accordingly, it is of very great industrial value to lower the temperature required for the sulfur trioxide decomposition reaction by using the sulfur trioxide decomposition catalyst of the present invention and let the sulfur trioxide decomposition reaction proceed at a substantive rate, for example, at a temperature of about 700° C. or less.

(Sulfur Trioxide Decomposition Catalyst—Composite Oxide)

As for the transition metal or rare earth element constituting the composite oxide of the sulfur trioxide decomposition catalyst of the present invention, an arbitrary transition metal or rare earth element, for example, a metal selected from the group consisting of: copper (Cu), chromium (Cr), titanium (Ti), zirconium (Zr), lanthanum (La), cerium (Ce), neodymium (Nd) and a combination thereof, may be used.

In the composite oxide of the sulfur trioxide decomposition catalyst of the present invention, the atom ratio between vanadium and at least one metal selected from the group consisting of transition metal and rare earth elements (at least one metal such as transition metal:vanadium) may be from 1:9 to 9:1, from 2:8 to 8:2, from 3:7 to 7:3, or from 4:6 to 6:4, that is, from 1.11:1 to 9:1, from 0.25:1 to 4:1, from 0.43:1 to 2.33:1, or from 0.67:1 to 1.5:1.

(Sulfur Trioxide Decomposition Catalyst—Composite Oxide—Embodiment Using Cerium)

In one embodiment, the transition metal or rare earth element constituting the composite oxide of the sulfur trioxide decomposition catalyst of the present invention is cerium (Ce) and the atom ratio (cerium:vanadium) between cerium and vanadium is from 0.4:1 to less than 1:1, from 0.5:1 to less than 1:1, from 0.6:1 to 0.99:1, from 0.6:1 to 0.95:1, from 0.7:1 to 0.95:1, or from 0.8:1 to 0.95:1.

As in this embodiment, when the transition metal or rare earth element constituting the composite oxide of the sulfur trioxide decomposition catalyst of the present invention is cerium, the catalytic activity can be enhanced by using cerium in a smaller atom ratio than vanadium.

Although not being bound by theory, this is considered to be related to the fact that the cerium-vanadium oxide has a zircon-type or scheelite-type structure represented by the formula CeVO₄ and in the structure above, cerium (Ce) is present in a trivalent (Ce^(III)) state (Jana Padezenik Gomilseek, etc., “X-Ray absorption study of CeO2 and Ce/V mixed oxide thin films obtained by sol-gel deposition”, Thin Solid Films, Volume 446, Issue 1, pp. 117-123), but in the use conditions of this composite oxide as a catalyst, cerium (Ce) is liable to take a tetravalent (Ce^(IV)) state.

Specifically, it is considered that in the use conditions of the cerium-vanadium oxide as a catalyst, the composite oxide is in a state of, as shown in FIG. 4( a), being somewhat positively charged {(Ce^(III) _(1-δ)Ce^(IV) _(δ)V^(V)O₄)^(δ+)} and this produces a state of oxygen (O₂) or oxygen ion (O²⁻) being enriched in the vicinity of the composite oxide surface. In this case, the equilibrium of the sulfur trioxide (SO₃) decomposition reaction shown by the following formula (B1-2) is believed to shift to the left side, allowing the sulfur trioxide decomposition reaction to scarcely proceed. SO₃(gas)→SO₂(gas)+½O₂(gas)  (B1-2)

That is, in the case of using the cerium-vanadium oxide as a catalyst, the conversion ratio from sulfur trioxide to sulfur dioxide is considered to be limited not by the reaction rate but by the equilibrium in the atmosphere near the composite oxide surface.

On the other hand, when cerium is used in a smaller atom ratio than vanadium, it is believed that the problem above is reduced due to lacking of cerium on the surface and at the same time, the portion deprived of oxygen corresponding to cerium becomes an oxygen defect, accelerating adsorption and decomposition of sulfur trioxide in the portion. However, if the ratio of cerium to vanadium is too small, this is considered to make it impossible to maintain the above-described structure, leading to reduction in the catalytic activity for the sulfur trioxide decomposition reaction. In this respect, it can be confirmed that when the ratio of cerium to vanadium is decreased, the above-described structure can be maintained until the ratio between cerium and vanadium (Ce:V) becomes about 0.5:1.

Incidentally, in the case of lanthanum (La) and neodymium (Nd) belonging to the rare earth similarly to cerium, the problem as encountered by cerium does not occur. This is considered because both lanthanum and neodymium stably have a trivalent oxidation state (FIG. 4( b)).

(Sulfur Trioxide Decomposition Catalyst—Composite Oxide—Production Process)

The composite oxide of the sulfur trioxide decomposition catalyst of the present invention can be obtained by an arbitrary method.

For example, the composite oxide may be obtained by mixing individual metal oxides constituting the composite oxide and firing the mixture. Also, in the case of selecting a salt of a transition metal element or the like and a vanadium salt to enable coprecipitation thereof, a precursor of the composite oxide is obtained by coprecipitation from an aqueous solution containing a salt of a transition metal element or the like and a vanadium salt, and the obtained coprecipitate is fired, whereby the composite oxide can be obtained.

Furthermore, when the sulfur trioxide decomposition catalyst of the present invention is a supported catalyst where the composite oxide is supported on a support, the support is caused to absorb an aqueous solution containing a salt of a transition metal element or rare earth element, dried and calcined, the support is then caused to absorb an aqueous solution of a vanadium salt, dried and calcined, and thereafter, the obtained support is fired, whereby the sulfur trioxide decomposition catalyst of the present invention can be obtained. Conversely, the support is caused to first absorb an aqueous solution of a vanadium salt, dried and calcined, the support is then caused to absorb an aqueous solution containing a salt of a transition metal element or the like, dried and calcined, and thereafter, the obtained support is fired, whereby the sulfur trioxide decomposition catalyst of the present invention can be obtained. Also, in the case of selecting a vanadium salt and a salt of a transition metal element or the like to enable coprecipitation thereof, the support is caused to absorb an aqueous solution containing both a vanadium salt and a salt of a transition metal element or the like, dried and calcined, and thereafter, the obtained support is fired, whereby the sulfur trioxide decomposition catalyst of the present invention can be obtained.

(Sulfur Trioxide Decomposition Catalyst—Support)

The composite oxide of the sulfur trioxide decomposition catalyst of the present invention may be supported on a support, whereby the surface area of the composite oxide can be increased and reduction in the surface area of the composite oxide during use can be suppressed. In this connection, the usable support includes a support selected from the group consisting of silica, alumina, zirconia, titania and a combination thereof.

Accordingly, for example, silica, particularly, a porous silica support having a pore structure, can be used as the support. In this case, the composite oxide is preferably supported inside the pore structure of the porous silica support. Also, in this case, the porous silica support preferably has a pore distribution such that the peak attributable to a gap between primary particles of silica is present in the range of a pore size of 5 to 50 nm, particularly a pore size of 5 to 30 nm, and the peak attributable to the pore structure inside the silica particle is present in the range of a pore size of 1 to 5 nm, particularly a pore size of 2 to 4 nm.

In the case of using a porous silica support having a pore structure in this way, the composite oxide is supported near the surface of the pore structure of the porous silica support, whereby sintering of the composite oxide particle can be suppressed. Although not being bound by theory, it is considered that in the composite oxide particle maintained in such a very fine state, not only the surface area of the catalyst increases about 100 times due to microparticulation of the catalyst but also the property of the catalyst surface is sometimes changed to improve the catalytic performance of the composite oxide.

Furthermore, it is considered that in the pore distribution of the porous silica particle having a pore structure, a two-dimensional distribution of pores is created and a vapor-phase gas having a high diffusion speed is supplied at a high speed from pores with a pore size of ten to several tens of nm to an active site having a wide surface area and a pore size of several nm, thereby increasing the chance of bringing the composite oxide particle into contact with sulfur trioxide, as a result, the catalytic performance is improved.

Incidentally, the porous silica support having a pore structure can be obtained by the method described in Kokai No. 2008-12382.

(Sulfur Dioxide Production Process)

The process of the present invention for producing sulfur dioxide includes decomposing sulfur trioxide into sulfur dioxide and oxygen by using the sulfur trioxide decomposition catalyst of the present invention. In this process, the sulfur trioxide decomposition catalyst of the present invention is used, whereby the decomposition can be performed at a lower temperature than in the conventional process of decomposing sulfur trioxide, for example, at a temperature of 800° C. or less, 750° C. or less, 700° C. or less, or 650° C. or less.

(Hydrogen Production Process)

The process of the present invention for producing hydrogen includes splitting water into hydrogen and oxygen, for example, splitting water into hydrogen and oxygen by an S—I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, or a Los Alamos science laboratory cycle process. Here, the process of the present invention includes decomposing sulfuric acid into water, sulfur dioxide and oxygen through a reaction represented by the following formula (X1) and out of the elementary reactions of formulae (X1-1) and (X1-2) which are an elementary reaction of the reaction represented by the following formula (X1), the elementary reaction of formula (X1-2) is performed by the process of the present invention for producing sulfur dioxide: H₂SO₄→+H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂  (X1-2)

That is, for example, in the S—I (sulfur-iodine) cycle process represented by the following formulae (X1) to (X3), the process of the present invention for producing hydrogen includes, out of the elementary reactions of formulae (X1-1) and (X1-2) which are an elementary reaction of the reaction of formula (X1), performing the elementary reaction of formula (X1-2) by the process of the present invention for producing sulfur dioxide: H₂SO₄→H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂  (X1-2) I₂+SO₂+2H₂O→+2HI+H₂SO₄  (X2) 2HI→H₂+½O₂  (X3) Total reaction:H₂O→H₂+½O₂

Also, for example, in the Westinghouse cycle process represented by the following formulae (X1), (X4) and (X5), the process of the present invention for producing hydrogen includes, out of the elementary reactions of formulae (X1-1) and (X1-2) which are an elementary reaction of the reaction of formula (X1), performing the elementary reaction of formula (X1-2) by the process of the present invention for producing sulfur dioxide: H₂SO₄→H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂  (X1-2) SO₂+2H₂O→H₂SO₃  (X4) H₂SO₃+H₂O+→H₂+H₂SO₄(electrolysis)  (X5) Total reaction:H₂O→H₂+½O₂

Furthermore, for example, in the Ispra-Mark 13 cycle process represented by the following formulae (X1), (X6) and (X7), the process of the present invention for producing hydrogen includes, out of the elementary reactions of formulae (X1-1) and (X1-2) which are an elementary reaction of the reaction of formula (X1), performing the elementary reaction of formula (X1-2) by the process of the present invention for producing sulfur dioxide: H₂SO₄→H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂  (X1-2) 2HBr→Br₂+H₂  (X6) Br₂+SO₂+2H₂O+→2HBr+H₂SO₄  (X7) Total reaction:H₂O→H₂+½O₂

In addition, for example, in the Los Alamos science laboratory cycle process represented by the following formulae (X1) and (X8) to (X10), the process of the present invention for producing hydrogen includes, out of the elementary reactions of formulae (X1-1) and (X1-2) which are an elementary reaction of the reaction of formula (X1), performing the elementary reaction of formula (X1-2) by the process of the present invention for producing sulfur dioxide: H₂SO₄→H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂  (X1-2) Br₂+SO₂+2H₂O+→2HBr+H₂SO₄  (X8) 2CrBr₃→2CrBr₂+Br₂  (X9) 2HBr+2CrBr₂→2CrBr₃+H₂  (X10) Total reaction:H₂O→H₂+½O₂

EXAMPLES Example A1

In this Example, a composite metal oxide (Cu—V—O) of copper (Cu) and vanadium (V) was used as the catalyst.

(Production of Unsupported Catalyst)

Copper oxide and vanadium oxide in an atom ratio of respective metals of 1:1 were ground in a mortar and thoroughly mixed, and the mixture was charged into an alumina-made crucible and fired at 750° C. over 12 hours, whereby the unsupported catalyst used as the catalyst was obtained. FIG. 1( a) shows the results of X-ray diffraction analysis (XRD) on the obtained unsupported catalyst.

(Production of Supported Catalyst)

A supported catalyst where a composite metal oxide is supported on a porous silica support having a pore structure was produced as follows.

(Production of Supported Catalyst—Production of Porous Silica Support)

The porous silica support for the supported catalyst was produced by the method similar to the method described in JP-A-2008-12382. That is, the porous silica support was produced as follows.

In 6 L (liter) of distilled water, 1 kg of cetyltrimethylammonium chloride was dissolved. The obtained aqueous solution was stirred over 2 hours to self-align the cetyltrimethylammonium chloride. Thereafter, tetraethoxysilane and aqueous ammonia were added to the solution having self-aligned therein cetyltrimethylammonium chloride and the pH of the solution was adjusted to 9.5.

In this solution, tetraethoxysilane was hydrolyzed over 30 hours to precipitate silica around aligned hexadecylamine, whereby a secondary particle composed of a primary particle having nanosize pores was formed and thus, a porous silica support precursor was obtained.

Thereafter, the obtained porous silica support precursor was washed with aqueous ethanol, filtered, dried and fired in an air at 800° C. over 2 hours to obtain a porous silica support.

The porous silica support obtained here had pores of around 2.7 nm attributable to the pore structure of silica and pores of over 10 nm attributable to the gap between primary particles of silica.

(Production of Supported Catalyst—Loading of Composite Metal Oxide)

The composite oxide was loaded on the porous silica support by a water absorption loading method. Specifically, first, an aqueous solution obtained by dissolving copper nitrate in water was prepared, and the support was caused to absorb the aqueous solution, dried at 150° C. and calcined at 350° C. for 1 hour. Next, ammonium metavanadate was dissolved in water, and the support was caused to absorb the resulting aqueous solution, dried at 150° C. and calcined at 350° C. over 1 hour. Finally, the obtained support was fired at 600° C. over 2 hours to obtain a composite oxide-supported porous silica support.

The amount of copper supported was set to 0.12 mol/100 g-support, and the amount of vanadium supported was set to 0.12 mol/100 g-support.

Example A2

In Example A2, a composite metal oxide (Cr—V—O) of chromium (Cr) and vanadium (V) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that in the production of an unsupported catalyst, the firing temperature was changed to 700° C.

FIG. 1( b) shows the results of X-ray diffraction analysis (XRD) on the obtained unsupported catalyst.

Example A3

In Example A3, a composite metal oxide (Ce—V—O) of cerium (Ce) and vanadium (V) was used as the catalyst. Here, an unsupported catalyst and a supported catalyst were produced in the same manner as in Example A1 except that in the production of an unsupported catalyst, the firing temperature was changed to 700° C.

FIG. 2( a) shows the results of X-ray diffraction analysis (XRD) on the obtained unsupported catalyst.

Example A4

In Example A4, a composite metal oxide (Zr—V—O) of zirconium (Zr) and vanadium (V) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that in the production of an unsupported catalyst, the firing temperature was changed to 700° C.

FIG. 2( b) shows the results of x-ray diffraction analysis (XRD) on the obtained unsupported catalyst.

Example A5

In Example A5, a composite metal oxide (Ti—V—O) of titanium (Ti) and vanadium (V) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that in the production of an unsupported catalyst, the firing temperature was changed to 600° C.

Example A6

In Example A6, a composite metal oxide (La—V—O) of lanthanum (La) and vanadium (V) was used as the catalyst. Here, a supported catalyst was produced in the same manner as in Example A1.

Example A7

In Example A7, a composite metal oxide (Nd—V—O) of neodymium (Nd) and vanadium (V) was used as the catalyst. Here, a supported catalyst was produced in the same manner as in Example A1.

Comparative Example A1

In Comparative Example A1, a composite metal oxide (Zn—V—O) of zinc (Zn) and vanadium (V) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that in the production of an unsupported catalyst, the firing temperature was changed to 700° C.

FIG. 2( c) shows the results of X-ray diffraction analysis (XRD) on the obtained unsupported catalyst.

Comparative Example A2

In Comparative Example A2, an oxide (Cr—O) of chromium (Cr) was used as the catalyst. Here, an unsupported catalyst and a supported catalyst were produced in the same manner as in Example A1 except that firing for forming a composite oxide was not performed in the production of an unsupported catalyst and the amount of chromium supported was set to 0.24 mol/100 g-support in the production of a supported catalyst.

Comparative Example A3

In Comparative Example A3, an oxide (Fe—O) of iron (Fe) was used as the catalyst. Here, an unsupported catalyst and a supported catalyst were produced in the same manner as in Example A1 except that firing for forming a composite oxide was not performed in the production of an unsupported catalyst and the amount of iron supported was set to 0.24 mol/100 g-support in the production of a supported catalyst.

Comparative Example A4

In Comparative Example A4, an oxide (Cu—O) of copper (Cu) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that firing for forming a composite oxide was not performed in the production of an unsupported catalyst.

Comparative Example A5

In Comparative Example A5, an oxide (V—O) of vanadium (V) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that firing for forming a composite oxide was not performed in the production of an unsupported catalyst.

Comparative Example A6

In Comparative Example A6, an oxide (Ni—O) of nickel (Ni) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that firing for forming a composite oxide was not performed in the production of an unsupported catalyst.

Comparative Example A7

In Comparative Example A7, an oxide (Co—O) of cobalt (Co) was used as the catalyst. Here, an unsupported catalyst was produced in the same manner as in Example A1 except that firing for forming a composite oxide was not performed in the production of an unsupported catalyst.

Comparative Example A8

In Comparative Example A8, a catalyst was not used.

Reference Example A

In Reference Example A, platinum was loaded on a γ-alumina support to produce a supported catalyst. Here, the amount of Pt supported was set to 0.5 g/100 g-support.

(Evaluation)

Using the fixed bed flow reactor shown in FIG. 1, the unsupported catalysts and supported catalysts of Examples, Comparative Examples and Reference Example were evaluated for the conversion ratio of sulfur trioxide decomposition reaction of the following formula (X1-2): SO₃→SO₂+½O₂  (X1-2)

Specifically, the conversion ratio of the sulfur trioxide decomposition reaction was evaluated as described below by referring to FIG. 3.

A quartz-made reaction tube 4 (inner diameter: 10 mm) was filled with, as a catalyst bed 10, 0.5 g of an unsupported catalyst or supported catalyst adjusted to 14 to 20 meshes, and nitrogen (N₂) (100 mL/min) and an aqueous 47 wt % sulfuric acid (H₂SO₄) solution (50 μL/min) were supplied to the lower stage of the quartz-made reaction tube 4 from a nitrogen supply part 1 and a sulfuric acid supply part 3, respectively.

The sulfuric acid (H₂SO₄) supplied to the lower stage of the quartz-made reaction tube 4 was heated in the lower and middle stages of the quartz-made reaction tube 4 to decompose into sulfur trioxide (SO₃) and oxygen (O₂) and flowed to the catalyst bed 10 (SO₃: 4.5 mol %, H₂O: 31 mol %, N₂: balance, gas flow rate in terms of 0° C.: 148.5 cm³/min, weight-flow ratio (W/F ratio): 5.61×10⁻⁵ g·h/cm³, gas hourly space velocity (GHSV): about 15,000 h⁻¹).

Here, in the quartz-made reaction tube 4, the lower stage was heated to about 400° C. by a heater 4 a, and the middle stage was heated to about 600° C. by a heater 4 b. Also, the upper stage of the quartz-made reaction tube 4 was initially heated to about 600° C. by a heater 4 c and once a stationary state was established, heated to 650° C.

After the upper stage of the quartz-made reaction tube 4 was heated to 650° C. by the heater 4 c, the outflow gas from the quartz-made reaction tube 4 was air-cooled and then bubbled through a 0.05 M iodine (I₂) solution, and sulfur dioxide (SO₂) was absorbed in the iodine solution. Iodometric titration of the iodine solution having absorbed therein sulfur dioxide was performed using a 0.025 M sodium thiosulfate (Na₂S₂O₃) solution, whereby the amount of sulfur dioxide absorbed was determined.

Also, the outflow gas after bubbling through the iodine solution was cooled with a dry ice/ethanol mixture, and the remaining sulfur dioxide and sulfur trioxide were completely removed with a mist absorber and silica gel. Thereafter, the amount of oxygen (O₂) was determined using a magnetic pressure oxygen analyzer (MPA3000, manufactured by Horiba Ltd.) and a gas chromatograph (GC8A, manufactured by Shimadzu Corporation, molecular sieve 5A, TCD detector).

The conversion ratio from sulfur trioxide (SO₃) to sulfur dioxide (SO₂) was calculated from the amounts of sulfur dioxide and oxygen determined as above.

Evaluation results of Examples, Comparative Examples and Reference Example are shown in Table 1 below.

TABLE 1 Conversion Ratio Without Support With Support (SiO₂) Catalyst (%) (%) Example A1 Cu—V—O 51.7 93.6 Example A2 Cr—V—O 45.9 — Example A3 Ce—V—O 26.9 67.9 Example A4 Zr—V—O 39.3 — Example A5 Ti—V—O 67.2 — Example A6 La—V—O — 80.0 Example A7 Nd—V—O — 93.6 Comparative Zn—V—O  9.8 — Example A1 Comparative Cr—O 40.5 53.3 Example A2 Comparative Fe—O 25.9 32.4 Example A3 Comparative Cu—O  6.2 — Example A4 Comparative V—O 22.3 — Example A5 Comparative Ni—O (no conversion) — Example A6 Comparative Co—O (no conversion) — Example A7 Comparative none (no conversion) — Example A8 Reference Pt — 84.3 Example A (γ-alumina support)

It is understood from Table 1 that compared with the catalysts of Comparative Examples, the catalysts of Examples have significantly preferred sulfur trioxide decomposition characteristics at a relatively low temperature of 650° C. Also, it is understood from Table 1 that the catalysts of Examples using no noble metal have sulfur trioxide decomposition characteristics equal to or higher than those of the catalyst of Reference Example using platinum that is a noble metal.

Incidentally, vanadium oxide, particularly vanadium pentoxide (V₂O₅), used in Comparative Example A5 was employed for accelerating the reaction of formula (C-2) of oxidizing sulfur dioxide to obtain sulfur trioxide in the method called a contact process of producing sulfuric acid through reactions represented by the following formulae (C-1) to (C-3): S(solid)+O₂(gas)→SO₂(gas)  (C-1) 2SO₂(gas)+O₂(gas)→2SO₃(gas)  (C-2) SO₃(gas)+H₂O(liquid)→H₂SO₄(liquid)  (C-3)

However, in Comparative Example A5 using vanadium oxide, the conversion ratio was significantly low compared with Examples.

Example B

In Example B, a composite metal oxide of cerium (Ce) and vanadium (V) was used as the catalyst.

(Production of Supported Catalyst)

A supported catalyst where a composite metal oxide is supported on a porous silica support having a pore structure was produced as follows.

(Production of Supported Catalyst—Production of Porous Silica Support)

The porous silica support for the supported catalyst was produced by the method similar to the method described in JP-A-2008-12382. That is, the porous silica support was produced as follows.

In 600 mL (milliliter) of distilled water, 100.0 g of cetyltrimethylammonium chloride was dissolved. The obtained aqueous solution was stirred over 1 hour to such an extent as not generating a bubble to self-align the cetyltrimethylammonium chloride. Thereafter, 69.35 g of tetraethoxysilane (sometimes called tetraethyl orthosilicate) was added to the solution having self-aligned therein cetyltrimethylammonium chloride, and the resulting solution was stirred for about 5 minutes until the system became uniform. Furthermore, 12.1 mL of 28% aqueous ammonia was added, and the resulting solution was stirred until the system became uniform. The pH of the obtained solution was 9.5.

This solution was left to stand for 1 day, and tetraethoxysilane in the solution was hydrolyzed to precipitate silica around aligned hexadecylamine, whereby a secondary particle composed of a primary particle having nanosize pores was formed and thus, a porous silica support precursor was obtained.

Thereafter, the obtained porous silica support precursor was dried at 120° C. overnight, calcined according to the diagram shown in FIG. 4, then fired in an air at 600° C. over 2 hours and further fired in an air at 800° C. over 2 hours to obtain a porous silica support.

(Production of Supported Catalyst—Loading of Composite Metal Oxide)

The composite oxide was loaded on the porous silica support by a water absorption loading method. Specifically, first, an aqueous solution obtained by dissolving cerium nitrate in water was prepared, and the support was caused to absorb the aqueous solution, dried at 150° C. and calcined at 350° C. over 1 hour. Next, ammonium metavanadate was dissolved in water, and the support was caused to absorb the resulting aqueous solution, dried at 150° C. and fired at 800° C. over 2 hours to obtain the supported catalyst of Example B.

The total amount of cerium and vanadium supported was set to 0.24 mol/100 g-support, and the molar ratio (Ce/V ratio) between cerium and vanadium was set to the value shown in Table 2.

(Evaluation)

The conversion ratio of sulfur trioxide decomposition reaction of the following formula (X1-2) was evaluated in the same manner as the evaluation in Example A1 and the like. SO₃→SO₂+½O₂  (X1-2)

Incidentally, the reaction temperature was set to 600° C., 650° C. or 700° C.

The evaluation results of Example B are shown in Table 2 below and FIG. 5.

TABLE 2 Achievement Ratio (%) Based on Ce/V Ratio Equilibrium Conversion (mol %) 600° C. 650° C. 700° C. 50 15.0 67.9 95.4 66.7 23.4 59.6 81.1 85 97.8 93.0 89.2 95 78.2 90.2 88.1 100 44.0 48.5 49.1

It is understood from Table 2 and FIG. 5 that in the case where the Ce/V ratio was 100 mol %, that is, where the molar ratio between cerium and vanadium was 1:1, even when the reaction temperature was raised to 700° C. from 600° C., the achievement ratio based on the equilibrium conversion was not significantly improved. On the other hand, in the case where the Ce/V ratio was 95 mol % or less, that is, where the molar ratio between cerium and vanadium was less than 0.95:1, even at a reaction temperature of 600° C. or 650° C., the achievement ratio based on the equilibrium conversion was improved.

It is also understood from Table 2 and FIG. 5 that at a reaction temperature of 600° C., the achievement ratio based on the equilibrium conversion was improved particularly when the Ce/V ratio is 85 mol % and 95 mol %.

FIG. 6 shows the results of X-ray diffraction (XRD) analysis on the cerium-vanadium oxide where the Ce/V ratio is 85 mol %. It is understood from the XRD analysis results that the XRD sensitivity of the composite oxide is low, the composite oxide basically has a CeVO₄ structure, that is, a zircon-type structure, the peak of (200) plane as the main peak is slightly shifted to the low angle side, and the peak on the higher angle side than the peak of (200) plane as the main peak, particularly, the peak of (420) plane, is large.

In this connection, low XRD sensitivity of the composite oxide is considered to reveal that the composite oxide is present as a fine particle, and shifting of the main peak to the low angle side is considered to reveal that the composite oxide has an oxygen defect.

Example C

In Example C, a composite metal oxide of cerium (Ce), lanthanum (La) or neodymium (Nd) and vanadium (V) was used as the catalyst.

(Production of Supported Catalyst)

A supported catalyst where a composite metal oxide is supported on a porous silica support having a pore structure was produced as follows.

(Production of Supported Catalyst—Production of Porous Silica Support)

The porous silica support for the supported catalyst was produced as in Example B.

(Production of Supported Catalyst—Loading of Composite Metal Oxide)

The composite oxide was loaded on the porous silica support by a water absorption loading method. Specifically, first, an aqueous solution obtained by dissolving nitrate of cerium, lanthanum or neodymium in water was prepared, and the support was caused to absorb the aqueous solution, dried at 150° C. and calcined at 350° C. over 1 hour. Next, ammonium metavanadate was dissolved in water, and the support was caused to absorb the resulting aqueous solution, dried at 150° C. and fired at 800° C. over 2 hours to obtain the supported catalyst of Example C.

The amount of each of cerium and vanadium supported was set to 0.12 mol/100 g-support.

(Evaluation)

The conversion ratio of sulfur trioxide decomposition reaction of the following formula (X1-2) was evaluated in the same manner as the evaluation in Example A1 and the like. SO₃→SO₂+½O₂  (X1-2)

Incidentally, the reaction temperature was set to 600° C., 650° C. or 700° C.

The evaluation results of Example C are shown in Table 3 below and FIG. 7.

TABLE 3 Achievement Ratio (%) Based on Equilibrium Conversion 600° C. 650° C. 700° C. Cerium 44.0 48.5 49.1 Lanthanum 55.2 73.8 90.7 Neodymium 56.3 81.5 86.7

It is understood from Table 3 and FIG. 7 that in the case of the catalyst of cerium-vanadium oxide, compared with the catalyst of lanthanum-vanadium oxide and the catalyst of neodymium-vanadium oxide, even when the reaction temperature was high, the enhancement of the achievement ratio (%) based on the equilibrium conversion was small. This suggests that in the case of using a cerium-vanadium oxide as the catalyst, the conversion ratio from sulfur trioxide to sulfur dioxide is limited not by the reaction rate under the control of the temperature but by the equilibrium, particularly, the equilibrium in the atmosphere near the surface of the composite oxide.

That is, it is estimated from Table 3 and FIG. 7 that in the catalyst of cerium-vanadium oxide, the composite oxide is in a state of, as shown in FIG. 4( a), being somewhat positively charged {(Ce^(III) _(1-δ)Ce^(IV) _(δ)V^(V)O₄)^(δ+)} and this produces a state of oxygen (O₂) or oxygen ion (O²⁻) being enriched in the vicinity of the composite oxide surface, allowing the sulfur trioxide (SO₃) decomposition reaction to scarcely proceed. On the other hand, in the catalyst of lanthanum-vanadium oxide and the catalyst of neodymium-vanadium oxide, both lanthanum and neodymium stably have a trivalent oxidation state and thanks to this state, the problem as encountered by cerium does not occur (FIG. 4( b)).

DESCRIPTION OF NUMERICAL REFERENCES

-   1 Nitrogen supply part -   3 Sulfuric acid supply part -   4 Quartz-made reaction tube -   4 a, 4 b, 4 c Heater -   10 Catalyst bed 

The invention claimed is:
 1. A sulfur dioxide production process, comprising decomposing sulfur trioxide into sulfur dioxide and oxygen with a sulfur trioxide decomposition catalyst, wherein the sulfur trioxide decomposition catalyst comprises a composite oxide of vanadium and at least one metal selected from the group consisting of transition metal and rare earth elements.
 2. The process as claimed in claim 1, wherein said at least one metal is selected from the group consisting of copper (Cu), chromium (Cr), titanium (Ti), zirconium (Zr), lanthanum (La), cerium (Ce), neodymium (Nd) and a combination thereof.
 3. The process as claimed in claim 1, wherein in said composite oxide, the atom ratio between said at least one metal and vanadium is from 1:9 to 9:1.
 4. The process as claimed in claim 1, wherein said at least one metal is cerium (Ce), and in said composite oxide, the atom ratio (cerium:vanadium) between cerium and vanadium is from 0.4:1 to less than 1:1.
 5. The process as claimed in claim 4, wherein in said composite oxide, the atom ratio (cerium:vanadium) between cerium and vanadium is from 0.8:1 to 0.95:1.
 6. The process as claimed in claim 1, wherein said composite oxide is supported on a support.
 7. The process as claimed in claim 6, wherein said support is selected from the group consisting of silica, alumina, zirconia, titania and a combination thereof.
 8. The process as claimed in claim 7, wherein said support is a porous silica support having a pore structure.
 9. The process as claimed in claim 1, wherein said decomposition is performed at a temperature of 800° C. or less.
 10. The process as claimed in claim 1, wherein said decomposition is performed at a temperature of 700° C. or less.
 11. A hydrogen production process comprising splitting water into hydrogen and oxygen, wherein the process comprises decomposing sulfuric acid into water, sulfur dioxide and oxygen through a reaction represented by the following formula (X1) and out of the elementary reactions represented by the following formulae (X1-1) and (X1-2), which are elementary reactions of the reaction represented by the following formula (X1), the elementary reaction of formula (X1-2) being performed by the process claimed in claim 1: H₂SO₄→H₂O+SO₂+½O₂  (X1) H₂SO₄→H₂O+SO₃  (X1-1) SO₃→SO₂+½O₂.  (X1-2)
 12. The hydrogen production process as claimed in claim 11, wherein the hydrogen production process is an S—I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, or a Los Alamos science laboratory cycle process. 